Forward and rearward bumpers are provided for cars in the automotive industry, which bumpers are capable, particularly in the event of car collisions at low speed, to absorb impact energy, so as to minimize damage to the vehicles. A conventional embodiment of a bumper includes a crossbeam for attachment of two crash boxes by way of which the crossbeam is fixed to the longitudinal load-carrying beams of a motor vehicle, as well as a plastic cover which contributes to the determination of the drag coefficient (cw value) and allows a visually appealing appearance.
Crossbeams of the type involved here are hitherto manufactured either from pressed pieces, from tubular members, or from steel sheet strands shaped through roll-forming. Among these manufacturing methods, roll-forming is the economically most attractive option due to the high shaping velocity and a low number of processing steps.
One problem associated with the roll-forming process resides in the fact that the crossbeams are made of steel sheet or steel sheet strands with uniform thickness and quality throughout. While for a crossbeam not all portions need to show, as a matter of demands made and type, the same thickness, it is, however, always necessary to utilize the steel sheets of a thickness which corresponds to the demands which are maximally made of a crossbeam. There exist, however, regions which do not require such a thickness. As a consequence, the material consumption for the crossbeams is unnecessarily large.
Based on this prior art, it is an object of the invention to provide, using the roll-forming process, on the one hand, a method for the manufacture of crossbeams as components of bumpers, which crossbeams are optimized as to weight, and, on the other hand, crossbeams which are optimized as to weight.
In the first solution variant, a steel sheet strand having throughout same thickness, width, and quality is provided on one of its broad sides over the entire length with sheet metal strips which extend in longitudinal direction in parallel relationship, and which with respect to the width of the steel sheet strand are, however, narrower. These sheet metal strips are secured on the steel sheet strand, whereby all known joining operations may be used, such as, for example, laser welding or continuous roller welding, gluing, punch riveting, and so forth.
The narrow sheet metal strips are secured on the steel sheet strands at those locations where the finished crossbeam should be configured to match the function as predetermined by the respective strength-crash-behavior with respect to the demands made of the crossbeam and with respect to type. Hereby, the particular advantage is realized that in accordance with the various functions, the sheet metal strips can be best suited as far as their thickness, and/or their width, and/or their quality are concerned. The steel sheet strand as the base element ensures hereby that the desired minimum functions are met with respect to stress. This means, it has the minimum thickness and minimum quality.
Such a composite element, also referred to as xe2x80x9cpatchwork blankxe2x80x9d, is subsequently shaped into the crossbeam through a roll-forming process in a direction transversely to its longitudinal extension. The sheet metal strips are then precisely positioned at those locations where the crossbeam is to be stiffened. This stiffening can, accordingly, be functionally met either by a corresponding thickness, and/or width, and/or quality of the sheet metal strips.
Within the scope of the second solution variant, initially steel metal strips of same or differing width, and/or thickness, and/or quality are joined along their parallel longitudinal sides to a strand-shaped composite element (xe2x80x9ctailored blankxe2x80x9d) with a planar broadside. As a consequence, a profile is realized on the other broadside of the composite element, as explained above with reference to the second solution variant.
When this composite element is subsequently shaped into the crossbeam by a roll-forming process in a direction transversely to its longitudinal extent, the areas, which protrude with respect to the thinnest steel sheet strips, are positioned precisely in those cross-sectional portions which should be reinforced at the crossbeam.
In the third solution variant, the steel sheet strand with same thickness, width, and quality throughout is exclusively shaped by roll-forming, on the one hand to the desired cross-section of the crossbeam and, on the other hand, is provided with reinforcing folds in predetermined cross-sectional portions. These reinforcing folds can be of double-layer or multi-layer configuration. They are always positioned in the cross-sectional profile of the respective crossbeam. They may extend parallel to its web or its legs or extend perpendicular with respect thereto.
Regardless as to which solution variant is chosen, the particular advantage of the invention resides in the considerable savings in weight, as compared to crossbeams produced conventionally by roll-forming, while yet always realizing uniform wall thickness and identical quality. As a result of the method according to the invention, a targeted arrangement of different thicknesses, widths, and materials implements a specific crash behavior which can be best suited. This method, furthermore, has the advantage that additional manufacturing steps which follow the shaping, such as, for example, incorporating reinforcements by welding, can be eliminated, since the reinforcements are already incorporated during the manufacture of the composite elements. This provides for substantial savings in tool costs and manufacturing costs.